Gold (Sanskrit Jval; Anglo-Saxon gold), Au (L. aurum, shining dawn); atomic weight 196.9665; atomic number 79; melting point 1064.43°C; boiling point 3080°C; specific gravity 18.88 (20°C); valence 1 or 3.
Known and highly valued from earliest times, gold is found in nature as the free metal and in tellurides; it is very widely distributed and is almost always associated with quartz or pyrite. It occurs in veins and alluvial deposits, and is often separated from rocks and other minerals by sluicing or panning operations. About two thirds of the world's gold output now comes from South Africa, and about two thirds of the total U.S. production comes from South Dakota and Nevada. The metal is recovered from its ores by cyaniding, amalgamating, and smelting processes. Refining is also frequently done by electrolysis. Gold occurs in sea water to the extent of 0.1 to 2 mg/ton, depending on the location where the sample is taken. As yet, no method has been found for recovering gold from sea water profitably. It is estimated that all the gold in the world, so far refined, could be placed in a single cube 60 ft. on a side.
Of all the elements, gold in its pure state is undoubtedly the most beautiful. It is metallic, having a yellow color when in a mass, but when finely divided it may be black, ruby, or purple. The Purple of Cassius is a delicate test for auric gold. It is the most malleable and ductile metal; 1 oz of gold can be beaten out to 300 ft2. It is a soft metal and is usually alloyed to give it more strength. It is a good conductor of heat and electricity, and is unaffected by air and most reagents. It is used in coinage and is a standard for monetary systems in many countries. It is also extensively used for jewelry, decoration, dental work, and for plating. It is used for coating certain space satellites, as it is a good reflector of infrared and is inert.
Gold, like other precious metals, is measured in troy weight; when, alloyed with other metals, the term carat is used to express the amount of gold present, 24 carats being pure gold.
The most common gold compounds are auric chloride (AuCl3) and chlorauric acid (HAuCl4), the latter being used in photography for toning the silver image. Gold has 18 isotopes; Au198, with a half-life of 2.7 days, is used for treating cancer and other diseases. Disodium aurothiomalate is administered intramuscularly as a treatment for arthritis. A mixture of one part nitric acid with three of hydrochloric acid is called aqua regia (because it dissolved gold, the King of Metals). Gold is available commercially with a purity of 99.999 + %.
For many years the temperature assigned to the freezing point of gold has been 1063.0°C; this has served as a calibration point for the International Temperature Scales (ITS-27 and ITS-48) and the International Practical Temperature Scale (ITPS-48). In 1968, a new International Practical Temperature Scale (ITPS-68) was adopted, which demanded that the freezing point of gold be changed to 1064.43°C.